The invention relates to a crystal oscillator, and in particular to a circuit and method for adjusting the output frequency of the oscillator without affecting the swing of the output signal.
The centre frequency produced by a crystal oscillator may be adjusted by changing or trimming the load capacitance of the crystal. An example of such an oscillator may be found in WO98/34338.
Trimming the load capacitance in this manner has an adverse effect in that the swing of the output signal is subject to change. To compensate for this change, it is known to use automatic gain control (AGC) to maintain a constant swing of the output frequency. However, using AGC introduces other effects, which may be undesirable, particularly when it is desired to manufacture the oscillator circuit as an integrated circuit.
Firstly, additional circuitry is required to provide AGC. This brings additional cost, and perhaps more importantly for portable devices, additional power supply requirements. The extra circuitry required by AGC includes a voltage level detector, a voltage reference, a filter and a comparator. The comparator is provided to compare the reference voltage with the detected level, and adjust the current within the oscillator core accordingly to achieve a constant swing.
Secondly, the addition of AGC circuitry forms a feedback loop which means that a settling time is introduced before a given output swing can be relied upon.
The aim of the present invention is to provide a trimmable crystal oscillator having a constant swing, but without having the disadvantages mentioned above.
According to a first aspect of the invention, there is provided a crystal oscillator circuit comprising an adjustable load capacitance, and means for controlling the load capacitance to obtain a particular centre frequency, and means for drawing a crystal core current in accordance with the chosen load capacitance.
According to another aspect of the present invention, there is provided a method of providing a desired output frequency from a crystal oscillator circuit, the method comprising the steps of adjusting the load capacitance of the crystal oscillator circuit to obtain the desired output frequency, and drawing a crystal core current in accordance with a chosen load capacitance.